1. Field of the Invention
The invention in general relates to the fabrication of layered superlattice materials, and more particularly to a chemical vapor deposition process especially useful for making layered superlattice oxides and precursors for the chemical vapor deposition process that may be used in making the layered superlattice oxides as well as other layered superlattice materials.
2. Statement of the Problem
Copending U.S. patent application Ser. No. 07/965,190 filed Oct. 23, 1992 now abandoned discloses that the layered superlattice materials discovered by G. A. Smolenskii, V. A. Isupov, and A. I. Agranovskaya (See Chapter 15 of the book, Ferroelectrics and Related Materials, ISSN 0275-9608, [V.3 of the series Ferroelectrics and Related Phenomena, 1984] edited by G. A. Smolenskii, especially sections 15.3-15) are far better suited for ferroelectric and high dielectric constant integrated circuit applications than any prior materials used for these applications. The application discloses that the layered superlattice materials spontaneously form layered superlattices, i.e. crystalline lattices that include alternating layers of distinctly different sublattices, such as ferroelectric and non-ferroelectric sublattices, and that the layered nature gives rise to significant properties, such as extremely low fatigue ferroelectrics. The layered superlattice oxides comprise complex oxides of metals, such as strontium, calcium, barium, bismuth, cadmium, lead, titanium, tantalum, hafnium, tungsten, niobium, zirconium, bismuth, scandium, yttrium, lanthanum and the lanthanides, antimony, chromium, molybdenum, vanadium, ruthenium, and thallium. Generally, each layered superlattice material will include two or more of the above metals; for example, strontium, bismuth and tantalum form the layered superlattice material strontium bismuth tantalate, SrBi.sub.2 Ta.sub.2 O.sub.9.
Numerous methods of fabricating the layered superlattice oxides are known. For example powders of the oxides or salts of the constituent metals may be mixed, pressed, and sintered at high temperature. See G. A. Smolenskii, V. A. Isupov, A. I. Agranovskaya, "Ferroelectrics of the Oxygen-Octahedral Type With Layered Structure ", Soviet Physics--Solid State, V. 3, No. 3, pp. 651-655(September 1961). Or solid oxides and/or carbonates of the constituent metals may be reacted at high temperature. See E. C. Subbarao, "Ferroelectricity in Mixed Bismuth Oxides With Layer-Type Structure", J. Chem. Physics, V. 34, 695 (1961) and J. Gopalakrishnan, A. Ramanan, C. N. R. Rao, D. A. Jefferson, and David J. Smith, "A Homologous Series of Recurrent Intergrowth Structures of the Type Bi.sub.4 A.sub.m+n-2 B.sub.m+n O.sub.3(m+n)+6 Formed by Oxides of the Aurivillius Family, Journal of Solid State Chemistry, Vol. 55, pp. 101-105 (1984). One layered superlattice oxide, bismuth titanate (Bi.sub.4 Ti.sub.3 O.sub.2), has been formed by RF sputtering, pulsed laser deposition, electron cyclotron resonance plasma sputtering, rapid quenching, and spinning of a sol-gel precursor onto a substrate followed by hydrolysis and heating. See P. C. Joshi, Abhai Mansingh, M. N. Kamalasanan, and Subhas Chandra, "Structural and Optical Properties of Ferroelectric Thin Films By Sol-gel Technique, "Appl. Phys. Lett., Vol 59, No. 10, November 91., Shu-Yau Wu, "A New Ferroelectric Memory Device, Metal-Ferroelectric-Semiconductor Transistor", IEEE Transactions On Electron Devices, August 1974, pp. 499-504, U.S. Pat. No. 5,146,299 issued to Donald R. Lampe et al. The Lampe patent also discloses fabricating a number of fluoride compounds that are layered superlattice materials (but not layered superlattice oxides) by using chemical vapor deposition with .beta.-diketonates and hydrogen fluoride used as precursors. However, Lampe does not recognize that the fluorides fabricated are layered superlattice materials, or even that they are related to bismuth titanate in any way except that all are ferroelectrics. Copending U.S. patent application Ser. No. 07/981,133, describes a method of fabricating layered superlattice materials in which each constituent metal is reacted to form a metal-carboxylate or metal-alkoxide, which metal carboxylates and/or metal-alkoxides are dissolved in a common solvent to form a precursor liquid; the precursor liquid is applied to a substrate by spinning, misting, or other application process followed by drying and annealing to form a solid thin film.
Chemical vapor deposition (CVD) is a well-known method of forming the layers in integrated circuits. See for example, M. J. Cooke, Semiconductor Devices, Prentice Hall, New York, N.Y. pp. 196-198, (1990). In particular simple metal oxides of the form ABO.sub.3 have been formed using a organic precursors. See for example, "Metalorganic Chemical Vapor Deposition of PbTiO.sub.3 Thin Films", by B. S. Kwak, E. P. Boyd, and A. Erbil, Applied Physics Letters, Vol. 53, No. 18, Oct. 31, 1988, pp. 1702-1704, Takuma Katayama, Masashi Fujimoto, Masaru Shimizu, and Tadashi Shiosaki, "Growth and Properties of PbTiO.sub.3 Thin Films Photoenhanced Chemical Vapor Deposition", Japanese Journal of Applied Physics, Vol. 30, No. 9B, September 1991, pp. 2189-2192, and "Photo-MOCVD of PbTiO.sub.3 Thin Films ",Journal of Crystal Growth, 115 (1991) 289-293, by the same authors. However, the use of CVD has not been reported for any layered superlattice oxide prior to the present invention. It is noted that in the one other disclosure that disclosed the use of CVD for layered superlattice materials, i.e. layered superlattice fluorides, the Lampe et al. patent referenced above, sputtering is recommended as the deposition process for the only layered superlattice oxide, bismuth titanate, discussed. Further the bismuth titanate formed is stated to be of a C-axis orientation which does not provide good ferroelectric properties as compared to an A-axis oriented layered superlattice material.
The fabrication process utilizing liquid or misted precursors disclosed in copending U.S. Pat. No. 5,423,285, provides layered superlattice materials with excellent properties. However, it requires that a common solvent be available in which a compound of each of the constituent metals in a given layered superlattice material is soluble. On the other hand, the CVD processes of the prior art do not provide good electrical properties even for the simple ABO.sub.3 type oxides. Thus it would be highly desirable to have a fabrication process that did not require a common solvent and at the same time results in an axis orientation which provides good electrical properties.
3. Solution to the Problem
The present invention solves the above problem by providing a CVD process of fabricating layered superlattice materials, and in particular, layered superlattice oxides, that results in small grains having mixed orientation and good electrical properties.
The invention also provides a method of fabricating an integrated circuit having at least one layered superlattice thin film deposited by a CVD process. Preferably the integrated circuit is a non-volatile memory.
The invention provides a method of making a CVD precursor utilizing methoxides, ethoxides, butoxides, propoxides and other compounds with which CVD precursors may be made for almost any layered superlattice material. The deposition preferably takes place on a substrate at between 400.degree. C. and 800.degree. C. This produces an amorphous or polycrystalline phase with relatively small grain boundaries.
The invention provides for a crystallization or recrystallization step after the layered superlattice thin film is formed in the CVD process. Preferably the thin film is crystallized or recrystallized in a rapid thermal processing (RTP) step. Preferably, the temperature is ramped over a range of from 1.degree. C. per second to 300.degree. C. per second and up to a temperature of from 500.degree. C. to 850.degree. C. for a holding period of from 5 seconds to 5 minutes. Alternatively, an oxygen furnace anneal may be used in place of or in combination with the RTP step.
The invention provides for depositing an electrode or contact on the layered superlattice material, followed by an oxygen furnace anneal at from 600.degree. C. to 850.degree. C. for a period of 15 minutes or more.
The invention also optionally provides for an ion implantation step after the formation of the layered superlattice thin film and prior to the RTP step. This ion implantation step creates ion damage on the surface which provides a large number of crystallization nucleation points of different orientations.
Preferably each of the heating steps, i.e. the CVD process, the RTP, and the anneal after contact formation takes place at the same or a higher temperature than the preceding heating step.
Preferably the invention also includes a step of prebaking the substrate in an oxygen furnace at a temperature of between 500.degree. C. and 1000.degree. C. prior to performing the CVD deposition step.
The methods described above result in layered superlattice materials with good electronic properties, such as high polarizability. This is believed to be due to a crystalline orientation that results in good electronic properties. The poor electronic properties of the prior art are believed to have been due to the surface stress of the substrate; it is believed that the surface stress causes a relatively poor C-axis phase to form. It is believed that the process reduces according to the invention reduces such stress and results in more A-axis oriented grains, and a better crystal structure. Numerous other features, objects and advantages of the invention will become apparent from the following description when read in conjunction with the accompanying drawings.